1. Field of the Invention
An embodiment of the present invention relates to a semiconductor device including a semiconductor element and a method for driving the semiconductor device.
2. Description of the Related Art
Storage devices including semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power is not supplied, and a non-volatile device that holds stored data even when power is not supplied.
A typical example of a volatile storage device is a DRAM (dynamic random access memory). A DRAM stores data in such a manner that a transistor included in a storage element is selected and electric charge is stored in a capacitor.
When data is read from a DRAM, electric charge in a capacitor is lost on the above-described principle; thus, another writing operation is necessary whenever data is read out. Moreover, since leakage current (off-state current) flows between a source and a drain of a transistor included in a storage element when the transistor is in an off state for example, electric charge flows into or out even if the transistor is not selected, which makes a data holding period short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power is not supplied, an additional storage device using a magnetic material or an optical material is needed in order to hold data for a long time.
Another example of a volatile storage device is an SRAM (static random access memory). An SRAM holds stored data by using a circuit such as a flip-flop and thus does not need a refresh operation. This means that an SRAM has an advantage over a DRAM. However, cost per storage capacity is increased because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power is not supplied.
A typical example of a non-volatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding electric charge in the floating gate. Therefore, a flash memory has advantages in that a data holding period is extremely long (almost permanent) and a refresh operation which is necessary in a volatile storage device is not needed (e.g., see Patent Document 1).
However, a gate insulating layer included in a storage element deteriorates by tunneling current generated in writing, so that the storage element stops its function after a predetermined number of writing operations. In order to reduce adverse effects of this problem, a method in which the number of writing operations for storage elements is equalized is employed, for example. However, a complicated peripheral circuit is needed to realize this method. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, high voltage is necessary for injection of electric charge into the floating gate or removal of the electric charge, and a circuit for generating high voltage is also necessary. Further, it takes a relatively long time to inject or remove electric charge, and it is not easy to perform writing and erasing at higher speed.